1. Field of the Invention
The present invention relates to image forming apparatus using, for example, an electrophotographic process, and more particularly, to an image forming apparatus that forms an image on a recording material, while transporting the recording material in an approximately vertical direction.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as copiers and printers, by evenly charging a photoreceptor, for example, formed in a drum-like shape (photoreceptor drum) and exposing the photoreceptor drum to light controlled, based on image information, an electrostatic latent image is formed on the photoreceptor drum. Then, by applying toner to the electrostatic latent image, a visible image (applied toner image) is developed. After a transfer unit transfers this applied toner image from the surface of the photoreceptor drum onto a recording material, a fixing device fixes the toner image to the recording material.
The fixing device for use in such an image forming apparatus is generally a combination of a fusing roller and a pressure roller pressed against each other. The fusing roller is made up of a cylindrical metal core member with an internal heat source and multiple layers formed surrounding the core including an elastic body layer and a release layer which are thermally resistive. The pressure roller is made up of a metal core member and multiple layers formed surrounding the core including a heat-resistive elastic body layer and a release layer made up of a heat-resistive resin film or a rubber film. In this configuration of the fixing device, the recording material carrying an unfixed toner image is allowed to pass between the rotating fusing roller and pressure roller. At this time, the unfixed toner image is heated and pressurized and thereby fixed to the recording material.
A configuration of the image forming apparatus is known in which the above fixing device is located vertically above the photoreceptor drum, a transport path is formed to transport the recording material supplied from a paper tray located vertically below the photoreceptor drum upward in an approximately vertical direction, the transfer unit located along the transport path transfers the toner image on the photoreceptor drum to the recording material, and the recording material is transported to the fixing device.
The image forming apparatus having this configuration features a very short transport path of the recording material and a most part of the transport path can be exposed only by opening one side of the apparatus. Thus, the image forming time from paper feed to paper ejection can be shortened and the recording material can be transported more readily. In the event of a paper jam, the jammed recording material can be removed more easily. Moreover, another merit is that the image forming apparatus can be designed to be installed in a smaller area.
However, in the image forming apparatus that transports the recording material upward in the approximately vertical direction (vertical transport), because the fixing device is located on the recording material transport path from down to up against the force of gravity, when the recording material is transported after the toner image has been fixed to it at the fixing device, the gravity is exerted on the recording material being ejected from the fixing device in an opposite direction on approximately the same line as the transport direction. For this reason, unlike a conventional image forming apparatus that transports the recording material horizontally, for the image forming apparatus with the vertical transport path, it is difficult to provide a stable support of the recording material being transported, utilizing the force of gravity, and the behavior of the recording material is liable to become unstable. Because of difficulty of stable support in the horizontal direction of the recording material being transported upward in the approximately vertical direction, nipped between and driven by the fusing roller and the pressure roller rotating, pressed against each other, the recording material becomes unstable in the horizontal direction. Consequently, the recording material may move in variable directions while being transported and wave-like flexure (so-called “paper cockle”) toward the forward direction is liable to occur in the recording material. In addition, the recording material is liable to touch any of the members surrounding the transport path. Depending on an angle at which the medium touches such a member, there is a fear of bending or folding of a lead edge corner of the recording material (so-called “dog-ear”).